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Old 01-30-2008, 02:13 AM   #1
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speedo off

Hey guys I have a question. I noticed for quite some time now that my speedo registers 10 mph lower than my actual speed from about 50mph and up. It seems about 5mph too low around 30mph. I confirmed this recently by installing my wife's GPS unit and it confirmed my speedo was off. Any ideas? I bought the car used about 16 months ago and it's always been off. The car had underdrive pulley's, flowmaster catback and a Steeda tri-ax shifter already installed when I got her if that hints at anything.

Thanx...
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Old 01-30-2008, 08:06 AM   #2
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Re: speedo off

I would check the rear gears and the speedo gear. See if either are not stock. I think the stock speedo gear is 23 tooth (not sure on color and double check that tooth count) and the rear gears should be 2.72. You can mark a spot on the drive shaft and then turn the rear tire after marking a spot on it. Turn it one full revolution and count the other and see how far it goes should be 2.73:1.

If your speedo is off your odometer is off as well.
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Old 01-31-2008, 12:06 PM   #3
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Re: speedo off

I'll check that. Don't the 1994-2004 GT's have 3.23 gears stock though?
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Old 01-31-2008, 01:10 PM   #4
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Re: speedo off

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Originally Posted by k28gt View Post
Hey guys I have a question. I noticed for quite some time now that my speedo registers 10 mph lower than my actual speed from about 50mph and up. It seems about 5mph too low around 30mph. I confirmed this recently by installing my wife's GPS unit and it confirmed my speedo was off. Any ideas? I bought the car used about 16 months ago and it's always been off. The car had underdrive pulley's, flowmaster catback and a Steeda tri-ax shifter already installed when I got her if that hints at anything.

Thanx...
Mine's the same with 3.73 gears, For every mile the odo shows, I've gone an extra tenth of a mile (judging by the mile markers on a highway) so every 10 miles = 1 mile/etc

I changed my speedo gear so its closer, but still shows slow

Oh and Blowing through the traps at 114 MPH my speedo usually shows between 80 and 90 MPH, I still don't know what's up with that
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Old 02-01-2008, 05:07 PM   #5
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Re: speedo off

94-95's came stock with 2:73 and had an upgrade option of 3:08.....and i think in 99 when they did the PI heads they changed to 3:27...i'm not sure what they did on the 96-98 though
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Old 02-02-2008, 06:53 PM   #6
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Re: speedo off

I have a 2004 so I'm assuming that a tuner would be the only way of correcting the speedo problem barring any actual mechanically problems, yes?
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Old 02-04-2008, 12:12 PM   #7
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Re: speedo off

i believe that is correct....the computer will then compensate for the gears and correct the speedo
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Old 02-04-2008, 01:42 PM   #8
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Re: speedo off

you can use a dallas speed cal unit on 99-04 manual cars. I had one on my 01.
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Old 11-22-2009, 08:25 PM   #9
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Re: speedo off

hi there,i have a 01 gt,3.73 gears, the speedo reads 5mph slow at 55,i dont see a speedcal,since my speedo reads slow,would my odometer be slow also?the car has white face cluster,too.
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Old 11-22-2009, 10:26 PM   #10
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Re: speedo off

I would try some kind of tuner to fix the problem.
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Old 11-22-2009, 10:26 PM   #11
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Re: speedo off

Oops, sorry. I kind of repeated what everyone else said.
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Old 11-23-2009, 08:26 AM   #12
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Re: speedo off

a speedo gear does help to fix it, but its not a 100% fix in most all cases as they do not make the correct gear to fix 3.73 o 4.10s for example so its basically a waste as it only gets you somewhat close (5-7mph off... so not really close)

A speedcal goes between the speedo gear and the computer (or cluster) and modifies the signal coming through it. They make them that work on gear setups (pre 01) and digital readouts (99+)

This is the best solution IF YOUR CAR WILL WORK WITH IT so check to make sure. Your odometer is also reading off as well so keep that in mind
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Old 11-23-2009, 06:34 PM   #13
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Re: speedo off

i checked the odometer today with a gps,says im going 1 tenth more for every mile,the speedo is reading 5-7 mph slow,i thought since the speedo is reading slow that the odometer would be reading slow?the speedo would read fast,right,if no speedcal was done.right?
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Old 11-24-2009, 12:01 PM   #14
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Re: speedo off

the odometer is a reflection of the speedometer so it will read off percentage wise just like the speedo
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Old 11-24-2009, 12:05 PM   #15
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Re: speedo off

Your speedo is off because of the gears. Either a tune or the speed calibrator will fix the problem. If the speedo is slow than the odometer should be slow as well, (you are going further than it is reading).

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Old 11-24-2009, 02:06 PM   #16
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Re: speedo off

my odometer is reading more than i am actualy going.but my speedometer is reading slower than what i am actualy going.
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Old 11-24-2009, 03:56 PM   #17
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Re: speedo off

my old 97 with 3.73 gears read 7mph fast vs the stock 2.73 gears
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Old 11-24-2009, 04:11 PM   #18
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Re: speedo off

my odometer is reading 1 tenth more for every mile
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Old 11-25-2009, 08:44 AM   #19
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Re: speedo off

thats about right then, mine was about the same with 2.73 to 3.73. I had the speedo gear that was for 3.27-3.55 gears i think~ but like said above it doesnt fix the issue.
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Old 11-27-2009, 02:42 PM   #20
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Question gears

i think my 3.73s are a little to much for highway ,2200rpm@65MPH,SOUNDS LIKE IM DOING 100MPH.DOES THIS SOUND ABOUT RIGHT?
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